Unmanned aerial vehicles, UAVs, either remotely piloted (RPVs) or semi- or fully autonomous, represent an increasingly important field of aircraft technology. When flying autonomous aerial vehicles it is of utter importance to secure in the route planning that the aerial vehicle will be able to reach an approved termination point. When a failure occurs in the UAV, such as engine failure or the like, the unmanned vehicle must terminate the flight at an approved termination point with an acceptable terrain or the like. Above all, it is necessary to fully exclude the risk of terminating or crashing the UAV in populated areas. This can be achieved by using termination points, wherein these points are predetermined as acceptable points. It is also important as stated above that the UAV is able to reach its termination point and that this is secured.
In EP 1 462 898 A2 is disclosed, how it is ensured that each leg of the flight route has a termination point that is reachable for the UAV, and how it is ensured that the UAV is able to reach that termination point. However, the process is carried out to a large extent manually.
However, in future applications it will probably become very complex and resource demanding to predetermine by hand termination points for each special assignment. In addition, access to acceptable termination points can regulate on how the break point routes, flight paths, should be set up as well as where one can fly. Especially, one wants to use route planners that automatically configure new routes.
To solve this problem predetermined and prestudied termination maps may be used. These maps should describe a flight path or route to home as well as give information concerning the lowest acceptable flight altitude in order to complete the termination of the UAV from an arbitrary point of the map. The generation of the map is based on the application “Emergency flight plan”.
Document US-A1-20040039498 discloses a method of constructing a density map which represents the terrain and wherein each cell of the map contains a density vector representing the density of portion of the terrain and is useful to determine the traversability of the terrain to mobile vehicles. Document U.S. Pat. No. 6,182,007 discloses a route planning algorithm for a vehicle that takes into account the aspect angle which the vehicle presents to threats in the environment. A route is determined through the environment by summing the map cost values of each grid cell though which the vehicle may travel. The terrain of the area is taken into account. Document US-A1-20040193334 discloses a method where a UAV after a failure determines to glide to a termination point that has been predefined prior to the mission. U.S. Pat. No. 4,538,229 discloses a method for examining single engine performance capabilities and developing alternate routes/paths for two engine aircrafts such that an attendant loss of altitude will not result in the aircraft dropping below a net flight path altitude and clearance distance with respect to the earth's surface along a given route between an originating point and a destination point. The object of the method is to maintain a safe elevation relative to the terrain during a failure in order to reach a predetermined destination for a full sized aircraft. However, these documents all disclose mission dependent complex methods that are not easily implemented in an UAV.
The general object of the invention is to provide a method that easily and very accurate determines alternate termination points that are reachable for an unmanned vehicle. The method should be easy to verify to achieve level A of the provisions laid down in RTCA-DO-178B. Even if a loss of performance occurs due to the indirect route to the termination point the loss is relatively small and does not incur a noticeable increase in lowest allowable flight altitude.